JPA (Hibernate) - Session/Transaction and lazy loading

Question

I have a Java EE project and the MySQL database is managed with an ORM. I worked a lot with Hibernate to learn what I am doing wrong and I think I understand session/transaction, but I do not know how I can solve this in my case/architecture.

I have a Project and a Person which are in a bi-directional n:m relation with a join table. The Project is the mapping-owner. Now I want to delete a Person which is connected to a Project.

So I thought, I can do this:

Person person = findPersonById(personId);
Set<Project> projects = person.getProjects();
Iterator<Project> iterator = projects.iterator();
while (iterator.hasNext()) {
    Project project = iterator.next();
    if (project.getPersons().contains(person)) {
        project.getPersons().remove(person);
        projectDao.updateProject(project);
    }
}
personDao.removePerson(personId);

But I get an error in this line:

Iterator<Project> iterator = projects.iterator();

But it seems it has to do with:

Person person = findPersonById(personId);

and:

public Person findPersonById(int personId) {
    SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
    Session sess = sessionFactory.getCurrentSession();

    Transaction tx = sess.beginTransaction();
    try {
        Person person = (Person)sess.createQuery("from Person where id = "+personId).list().get(0);
        tx.commit();
        return person;
    }
    catch (IndexOutOfBoundsException ex) {
        return null;
    }
}

Here I make a transaction and only get the person object, but no corresponding projects. And the iterator will try to get them, but can not, because my transaction/session is closed. This is because of my architecture:

I have: IPersonService -> IPersonImplementation -> IPersonDao -> IPersonDaoImplementation and only the method in IPersonDaoImplementation opens and closes the transaction. So in my ServicePersonImplementation I can not do several DB related stuff (I do not want to have FetchType.EAGER in my @ManyToMany annotation, because this would load to much not needed stuff). Because once it returns from my DaoImplementation I can not perform other actions on the returned person (in my case get all projects which are combined with this person).

So what can I do in my case?

Best Regards Tim.

Update: PersonDaoImpl, remove method:

public void removePerson(int personId){
    Person p = findPersonById(personId);

    SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
    Session sess = sessionFactory.getCurrentSession();

    Transaction tx = sess.beginTransaction();
    sess.delete(p);
    sess.flush();
    tx.commit();
}

Trace of the error:

ERROR: org.hibernate.LazyInitializationException - failed to lazily initialize a collection of role: com.testdomain.testproject.domain.Person.projects, no session or session was closed org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.testdomain.testproject.domain.Person.projects, no session or session was closed at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:383) at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:375) at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:368) at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPersistentCollection.java:111) at org.hibernate.collection.PersistentSet.iterator(PersistentSet.java:186) at com.testdomain.testproject.dao.impl.PersonDaoImplHibernate.removePerson(PersonDa oImplHibernate.java:71) at com.testdomain.testproject.service.impl.PersonServiceImpl.removePerson(PersonServiceImpl.java:88) at com.testdomain.testproject.service.PersonTest.testRemovePerson(PersonTest.java:180) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20) at org.junit.runners.BlockJUnit4ClassRunner.runNotIgnored(BlockJUnit4ClassRunner.java:79) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4Cla ssRunner.java:71) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184) at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28) at org.junit.runners.ParentRunner.run(ParentRunner.java:236) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:46) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunne r.run(RemoteTestRunner.java:390) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197) Failed deleting Person: failed to lazily initialize a collection of role: com.testdomain.testproject.domain.Person.projects, no session or session was closed

Answer 1

What you really want to do is push all of the Person removal code down into the PersonDao's remove() method. Inside that method, you want to do the following:

• begin transaction
• Find Person instance by the id
• Get its Project set and remove it from all of them
• Remove the Person instance itself
• commit transaction

In JPA, you shouldn't need to do anything more than that, however I believe in straight Hibernate you may need to saveOrUpdate then changes made to each Project as well as remove the Person.

So your code might look something like this:

public void removePerson(int personId) {
  SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
  Session sess = sessionFactory.getCurrentSession();
  Transaction tx = sess.beginTransaction();

  try {
    Person person = (Person)sess.createQuery("from Person where id = "+personId).list().get(0);
    Set<Project> projects = person.getProjects();
    for (Project project : projects) {
      if (project.getPersons().contains(person)) {
        project.getPersons().remove(person);
        sess.saveOrUpdate(project);
      }
    }

    sess.delete(person);
    tx.commit();
  }  catch (Exception e) {
     System.err.println("Failed deleting Person: "+e.getMessage());
  }
}

(Note: this is roughly closer to pseudo code and has not been tested inside an actual IDE)

Answer 2

You have two options here really. The first one is to perform this in your service/DAO implementation.

projectDao.removePersonFrom(Person personToRemove, Project projectToRemoveFrom);

personDao.removeProjectFrom(Project projectToRemove, Person personToRemoveFrom);

I usually have a Model class associated with a view and sometimes I make certain methods in them transactional (I use Spring) to do this kind of stuff.

--edit--

Inside the daos you create your session/transaction and then you can do the code that fails. The code you have posted should work fine.